Plastic lenses have characteristic properties that glass lenses do not have, such as lightweight, safety, high workability and fashionability, and are now mainstream in the spectacle lens field. However, for example, diethylene glycol bisallyl carbonate resin lenses which are commonly used have a refractive index of 1.50 which is lower than that of glass and become thick at the periphery. Therefore, in the field of resin lenses, a synthetic resin lens having a higher refractive index is used to reduce the thickness.
Meanwhile, as plastic lenses have a disadvantage that they are easily scratched, a silicone-based hard coat layer is formed on the surface to improve the disadvantage. This silicone-based coat layer is formed by preparing a coating composition comprising silica fine particles, an organic silicon compound having a hydrolysable group, a curing catalyst, an acid aqueous solution and a water-soluble solvent as main components (may be referred to as “low refractive index coating composition” which comprises silica fine particles hereinafter), applying the coating composition to the surface of a plastic lens, and heating to cure the coating film and evaporate the contained solvent (refer to JP-B 57-2735).
However, when a hard coat layer is formed on a high refractive index plastic lens having a refractive index of 1.60 or more by using the above low refractive index coating composition, an interference fringe is produced due to the difference in refractive index between the plastic lens and the hard coat layer, resulting in a poor appearance.
To solve this problem, various studies are now under way. For example, there is known a coating composition comprising a composite metal oxide containing Sb, Ti, Zr or Sn having a high refractive index in place of the silica fine particles which are one of the components of the coating composition (JP-A 5-264805). This coating composition can be advantageously used for high refractive index plastic lenses. However, since a hard coat layer formed from a coating composition comprising titanium oxide deteriorates due to the photocatalytic activity of titanium oxide, there is room for the improvement of weather resistance. Therefore, a high refractive index coating composition comprising rutile type titanium oxide has been developed to reduce the photocatalytic activity of this titanium oxide (refer to JP-A 11-310755).
Although the above weather resistance is improved by the above high refractive index coating composition comprising rutile type titanium oxide, it was found through studies conducted by the inventors of the present invention that a hard coat layer obtained from the above coating composition becomes clouded, resulting in a poor appearance.
Further, performance required for the hard coat layer is becoming higher and a coating composition having the following performance in addition to the above performance is desired. Stated more specifically, an antireflection film may be formed on a hard coat layer formed from the coating composition to enhance the function of a plastic lens, and the alkali cleaning of the lens may be carried out before the formation of this antireflection film. Therefore, the lens having a hard coat layer must have such performance that the hard coat layer hardly peels off by alkali cleaning (this performance may be referred to as “chemical resistance” hereinafter).
Further, there is a case where a plastic lens comes into contact with hot water according to its application purpose. Even in this case, performance that the hard coat layer does not crack is required (this performance may be referred to as “hot water resistance” hereinafter).
Moreover, when a plastic lens expands by heat, performance that the hard coat layer can follow this expansion and does not crack is required (this performance may be referred to as “heat resistance” hereinafter). Especially heat resistance is required for the hard coat layer formed as described above and also when the hard coat layer is to be formed. That is, when the hard coat layer is formed on a plastic lens, heating is required to cure the coating composition. At this point, there is a case where the hard coat layer cracks due to the shrinkage of the hard coat layer and the expansion of the plastic lens by heating. Therefore, this cracking must be suppressed. Further, at the time of processing based on the specifications of a plastic lens, a coating composition having excellent moldability is desired.